This invention relates generally to electron guns and in particular to a correcting magnetic system for isolating the individual magnetic fields in closely adjacent electron gun assemblies.
Electron guns (e-Guns) are extensively used for metal evaporation and vacuum deposition upon substrate materials and involves the magnetic focusing of an intense electron beam from an electron emitter into a crucible containing the evaporant source material to be evaporated. The focusing of the electrons is usually accomplished by a transverse magnetic field generated at the end of the crucible opposite the electron emitter, but a more modern and improved system involves the use of vertical side magnets aligned along the sides of the e-Gun assembly and in the area of the beam path to form a transverse field to guide the electrons into the crucible.
The e-Gun assembly is formed of a rectangular block of nonferrous metal with a centrally located circular well in the top surface forming the crucible. The electron source is an emitter located at one end and magnetic flux generating permanent magnets or electromagnets suitably positioned to focus the electron beam into the crucible. When two or more e-Gun assemblies are brought together, there is the danger that their magnetic flux fields will interact to destroy the focus of their beams into the crucibles. This becomes a serious problem if a plurality of e-Guns are closely adjacent each other to use for vapor mixing or the simultaneous evaporation and deposition of multiple component layers in a single vacuum cycle. Normally for simultaneous evaporation, it is advantageous to position two or more e-Guns close together and equidistant from the substrate for vapor mixing or multi-layer deposition to assure the same degree of vapor mixing of evaporants or a greater coating rate and thus a higher efficiency of operation.
The problem of flux interaction and massive skewing of the magnetic fields in closely adjacent e-Gun assemblies has been recognized for many years and is discussed in my U.S. Pat. No. 3,475,542 of November 1969. To reduce this problem, present technique requires that the assemblies are staggered in a line in what is referred to as an "opposed-parallel" mode in which all North pole magnets along one side of an assembly are adjacent the North poles on the side of the next adjacent assembly, and South poles are adjacent South poles. In this configuration the adjacent poles of adjacent assemblies are oppositely polarized so that there is minimal distortion of the flux path of each assembly. This allows adjacent e-Gun assemblies to be spaced to within about 1/2 inch between pole pieces. However, a problem with this configuration is that, although a long linear array of assemblies can be made, the high voltage (e.g. 10 Kv.) supplied to the electron emitters at the ends of the staggered adjacent assemblies requires HV conductors and HV leads on both sides of the set of crucibles. This larger area of HV components makes for a markedly larger number of High Voltage arc-down and gaseous discharges, and HV insulators over a larger area of the chamber floor.
Placing the electron beam assemblies in a strict parallel mode with all emitters along one side of a multi-assembly evaporation unit places the magnets and poles in positions so that the magnetic sidewall poles of adjacent assemblies not only do not repel each other as in the opposite-parallel mode, but they attract each other and cause massive skewing of the fields and electron beam paths. The e-Gun magnetic fields act as one big magnet with field lines passing from an outside pole of one assembly over and above all intermediate poles to the outer pole of the most distal pole in the parallel assembly. The presence of the field lines above the adjacent parallel gun assemblies causes a flattening of the normal field lines in that region where the electrons fly as well as a change in the field line direction and the general electron focusing and skewing accuracy.
For reasonable operation in such a parallel mode, the adjacent assemblies must be separated by nearly the width of an assembly to eliminate such interaction
The system to be described eliminates the need for such physical separation and permits the accurate focusing of the electron beam in each of several assemblies in a parallel-parallel mode with adjacent assembly spacing of only about 1/4 inches.
Briefly described, the invention is for a fence magnet that is wafer thin to be positioned between aligned adjacent electron evaporation assemblies. The adjacent assemblies are normally spaced about 1/4 inches and the wafer-like magnet, having a thickness of about 1/8 inch is centrally positioned in the gap between and parallel with the sidewalls of the assemblies. The magnets are polarized through the thin direction of the wafer so that all of one surface will be a North polarity and the opposite surface is South polarity. The side surfaces, being oppositely polarized, are positioned in the gap so that their polarity repels that of the adjacent assembly pole. This prevents cross action of fields between adjacent assemblies and allows the assembly magnets to properly focus the electron beam into the crucible.